An ignition system, which is used for igniting, for example, a fuel-air mixture within a combustion chamber, includes an ignition plug mounted to an internal combustion engine or the like, and a power supply which applies electric energy to the ignition plug. In general, such an ignition plug includes an insulator having an axial hole extending in the axial direction, a center electrode inserted into a forward end portion of the axial hole, a metallic shell provided around the insulator, and a rod-shaped ground electrode fixed to a forward end portion of the metallic shell. A spark discharge gap is formed between a distal end portion of the ground electrode and a forward end portion of the center electrode. When electric energy is supplied from the power supply to the spark discharge gap, spark discharge is produced, whereby the fuel-air mixture or the like is ignited (see, for example, Japanese Patent Application Laid-Open (kokai) No H5-343157 “Patent Document 1,” etc.).
Also, there has been proposed a technique of improving the ignition performance and durability of such an ignition plug by providing a protrusion formed of a noble metal alloy or the like on the distal end portion of the ground electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No 2009-158339 “Patent Document 2,” etc.).
In recent years, a lean burn engine, a direct injection engine, a low emission engine, etc. have been actively developed so as to cope with emission control and improve fuel consumption. In such an engine, a higher ignition performance is needed.
The present invention has been accomplished in view of the above circumstances, and an object of the invention is to provide an ignition system which sufficiently utilizes the energy which is output from a power supply so as to produce spark discharge, to thereby realize excellent ignition performance.